Temperature compensation of voltage regulators has long been a problem. The reference voltage of regulators has typically been produced by adding a BJT base emitter junction voltage (VBE) to another derived voltage which is proportional to absolute temperature (PTAT). The simplest implementation of this method to achieve zero temperature co-efficient (ZTC) produces a reference voltage of 1.26 volts which is the popular bandgap voltage. With an adequate supply voltage and additional amplification circuitry this reference can be multiplied up or divided down to produce any value of regulated ZTC voltage.
These circuits however are not suitable for low supply voltage operation (1.3 volts or less) which is often required in battery operated circuits as there is not enough voltage to operate the simple band gap reference let alone the amplification circuitry required for regulation. In order to overcome this problem complicated circuitry has been used to implement essentially the same idea. This is accomplished by combining the right proportions of a VBE to produce some desired ZTC reference voltage which is less than the bandgap voltage.